In Cerise Park, Y/N is currently train his new Pokemon to prepare for the World Coronation Series. Rookidee is sparing with Treecko and Sandile is sparing with Rockruff. Gible training with Totodile. Emolga is sparing with Monferno. Riolu is sparing with Dragonite. While Y/N watch them train.

Y/N: Now Treecko use Bullet Seed! Rookidee dodge it!

Treecko: Treecko!

Rookidee: Rookie!

Treecko fired Bullet Seed at Rookidee but flew up into the air to dodge it. 

Y/N: Now Rookidee use Air Slash! Treecko use Quick Attack to dodge!

Rookidee: Dee!

Treecko: Treecko!

Rookidee attacks using Air Slash at Treecko who dodges using Quick Attack.

Y/N: Great job you two! Let's take a break.

Suddenly they heard a scream coming from inside the lab. 

Y/N: That sounded like Ren! 

Y/N and Riolu ran inside the lab and up the stairs where they met up with Goh and Scorbunny. They found Ren who is slumped against a wall and paralyzed with fear. 

Goh: Are you okay? 

Y/N: What happened to you?

Ren: (scared) I've never been more scared! It was that... Shiny Gengar.

Y/N and Goh: Gengar's back?

Ren nods with tears in his eyes.

Y/N: You know what that mean!

Goh: Another chance to catch it!

Y/N: I'm gonna catch it first!

Goh: Hold on. That my line!

Riolu: Olu!

Scorbunny: Scorbun!

Ren: (scared) Don't leave me all by myself!

Y/N and Goh begin searching all over there second floor to found the Shiny Gengar but couldn't find it. 

Goh: Not a sign. Whenever Gengar appears, the temperature is supposed to go down, right?

Goh notices Y/N look concerned about something.

Goh: Y/N, what's wrong?

Y/N: It's nothing I

Suddenly they heard Chrysa screaming from downstairs and they ran down the stairs to see what happened. They got to the lab and saw the room was a complete mess. Papers were scattered all over the floor, books were of the shelter, and the furniture was knocked over. 

Goh: Chrysa, was that Gengar?

Chrysa: Yes. It just appeared and started attacking all of a sudden.

Y/N: Do you know where it went?

Chrysa: I'm not sure. It just disappeared after that.

Y/N: It couldn't have gone far. Let's try looking for it around here.

Goh: Right.

Behind them Gengar who is invisible uses Psychic to levitate a bunch of books and aimed them all at Riolu. Y/N used his Aura to sense Gengar and turned around to see the floating being thrown at Riolu. Y/N quickly jumped in front of Riolu using his back to block the books. After Y/N blocked the books, Gengar become so frustrated that it made its body visible. 

Goh: It's Gengar!

Gengar: Gengar!

The Shiny Gengar continued the attack by firing a barrage of Shadow Balls at everyone, forcing everyone to hide behind cover. 

Y/N: Gengar stop it!

Gengar then disappeared after Y/N ordered it to stop and everyone came out of hide. 

Goh: It ran away.

Professor Cerise, Ren, and Yamper enter the room.

Yamper: Yam Yam

Professor Cerise: Looks like it really made a mess this time.

Everyone then helped clean up the lab and put all the furniture back in place. After they finished cleaning up the lab, they all sat down to wonder why Gengar is haunting the lab.

Ren: I'd sure like to know what that Gengar's after.

Chrsya: There's got to be a reason why Gengar keeps coming around.

Professor Cerise: Before I bought this place, I was told there was a rumor about it being a haunted house.

Goh: A haunted house?

Professor Cerise: The real estate agent said that ever since the original owner left eerie things happened whenever a potential buyer would visit. I'd say the haunting was just Gengar playing pranks.

Y/N:-_- So you thought it was a good idea to buy a haunted house.

The Professor nervously scratched his head and lightly chuckled.

Professor Cerise: Well I figured lights might go out and doors might close by themselves, and it really didn't bother me that much. It never went on for very long so I figured it was just fine.

Chrsya: That's our Professor for you.

Goh: I suppose that since Gengar didn't get much of a reaction out of you, it just decide to give up.

Y/N: But it doesn't explain why Gengar is still staying here. 

Goh: That is indeed a mystery! First, Ren and Magnemite were attacked in the basement. That can only mean...

Ren: (shocked) That's my fault?

Chrsya: That may be part of it, but since the rest of us got here and all the Pokemon came, this place has become a lot more lively, and perhaps the issue  is that Gengar doesn't like it very much.

Ren: You know, I think you've hit the proverbial nail on the head!

Y/N is deep thought wondering why Gengar is here and why is it so upset at us. Goh noticed Y/N thinking about something.

Goh: Is something on your mind?

Y/N: I'm wondering why Gengar is here in the first place and why did it attack us? It's like it's trying to force us to leave. 

Goh: You're right. There's have to be a reason why Gengar is staying here and why it's acting like this. 

Y/N: Let's check out the basement. Since Ren found Gengar hiding down there, maybe we can find some answers. 

Goh: Yeah good idea.

Meanwhile, from a distance Team Rocket were outside a window listening their conversation.  

Jessie: Did you hear that?

James: A Shiny Gengar would be the perfect addition to our team.

Meowth: If we get that Shiny Gengar to join us the boss will definitely promote us!

Jessie: Then it's settled! We'll be the ones capture Gengar!

Y/N, Riolu, Goh, and Scorbunny went down to the basement to investigate for clue about Shiny Gengar. 

https://youtu.be/k-M0hSko3JU

The light in the basement was working so they had go down with flashlights. The basement full of old equipment and dusty furniture. Y/N and Goh searched the room to find any clues. After a few minutes of searching, Y/N noticed a blank covering something in the corner. 

Y/N: Hmm what's that?

Y/N walk over to the corner and move the blanket, revealing it to be an old dusty wooden trunk.

Goh: A trunk? 

Y/N: Let's open it and find out.

Meanwhile, Gengar is hiding in the shadows watching Y/N, Riolu, Goh, and Scorbunny. The two trainers opened the trunk and saw it was full of old belonging. 

Y/N: Someone been keeping all this stuff down here.

Goh: But who does this stuff belong too.

Y/N then find a dusty picture frame next to a Poke Ball. Y/N picked it up the dusty picture. He wiped the dust off the picture and he widen his eyes. 

Y/N: No way!

In the picture, he saw an old man with a Shiny Haunter right beside him.

Goh: That Haunter! Could it be...

Gengar saw Y/N and Goh open the old trunks and was so furious that it emerged from the shadow, surprising Y/N and Goh.

Gengar: Gengar!

Y/N: It's back!

Out of rage Gengar fired a giant Shadow Ball 

and launched it the two trainers and their Pokemon. They dodged the giant Shadow Ball then Gengar attacks them with Dark Pulse, causing them to dodge again.

Y/N: Calm down! We don't want to fight you!

Goh: We just want to find out why you're so mad at us!

Riolu: Rio!

Scorbunny: Scorbunny!

Gengar: Gen!?

Gengar stopped attacking and Y/N showed it the picture.

Y/N: Gengar, this is haunter in this picture is you, isn't it? Does that mean is your trainer?

Gengar looked at the picture for a few seconds and became sadden after it look at the old man. 

Y/N notice Gengar's sadness and tried to comfort it.

Y/N: Gengar...

Gengar then disappeared before Y/N could talk to it. Y/N and Goh looked concerned for Gengar. Goh and Scorbunny went up stair to tell the Professor but then Y/N noticed a folded  paper that was inside the trunk. Y/N picked up the paper and put it in his pocket.

Timeskip

After discovering the old trunk in the basement, Y/N and Goh showed Professor Cerise the picture and explained what happened.

Professor Cerise: I see. The man in the picture must be Gengar's trainer.

Goh: We found it in an old trunk in the basement. It must belong to that man too.

Y/N: That means Gengar's trainer must have been the previous owner of the building.

Professor Cerise: It is a possibility. That might explain the reason why Gengar is here.

Y/N: Professor, do you know exactly what happen to the previous owner?

Professor Cerise: Well I'm not entirely sure. According to the rumor, he left one day, three years ago.

Y/N: He been gone for three years!

Goh: Do you think he moved and left Gengar?

Y/N: Maybe but I doubt he would move and just abandon Gengar. Not only that but why would he leave without taking his stuff too.

Goh: That's good point.

Y/N: I also found this in the trunk too.

Y/N took the folded paper out of his pocket and unfolded it.

Goh: What is it?

Y/N: It's a note. 

Y/N and Goh started to read the note. Meanwhile outside the Cerise lab entrance, Gengar is sitting on steps with a depressed expression. Seeing its trainer in the picture caused the Ghost type to remember all the happy memories they had together.

(Flashback)

On a rainy night, Shiny Haunter is wandering around looking for food and shelter from the rain. It came across a large building and decide to sneak in to get out of the rain. While inside Hauter started looking around for food but then the light turn on and the owner spotted it. Haunter quickly turns invisible to hide.

Old man: There no need to be scared. I won't hurt you.

The Shiny Haunter reveals itself.

Haunter: Haunt

Old Man: You must be hungry. How about I get you something to eat?

Haunter: (nodding) Haunter!

After meeting for the first time Haunter and the owner become the best of friends. They both enjoyed each others company so much that Haunter decided to be the old man's Pokemon. They did everything together such as going on walk, play outside, and have battles. Eventually the trainer decided that Haunter should evolve so he traded Haunter to trigger its evolution into Gengar and trade it back to himself.  After a few month Gengar had to help take care of his trainer due to his aging. Eventually the owner had to leave to go to the Hospital and he left Gengar in charge of his home.

Old Man: Gengar, I have to go to the Hospital for an appointment. I need you to stay here to look after the building.

Gengar: Gengar

Old Man: (petting Gengar on the head) Don't worry.  I'll try to get back as soon as can, okay. I promise when I go back we'll have a lot more fun later.

Gengar: (excited) Gengar!

Old Man: (starting to walk away) I'll be right back.

Gengar waved its trainer goodbye and patiently waited for his return. Unfortunately the old man had been gone all day but Gengar still waited for him. Days turned into months and months turn into years. It been 3 years since the trainer left and Gengar still waiting for his return.  When the building was put up for sale, Gengar scared away anyone who tried to buy it. The Ghost type wanted to protect its trainer's home and refused to let anyone take it. Despite waiting for three years, Gengar never lost hope and waited for the day its trainer would return.

Music stops

(Present)

 Team Rocket then showed up and interrupts Gengar's peacefully contemplation.

Jessie: Just a moment. 

Gengar: Gen?

Jessie: I have an idea.

James: Why not join our team? We need a powerful Pokemon like you on our side.

Meowth: With your help, taking over the Galar region will be a breeze! So what do say?

Gengar: (refusing) Gengar! Gengar!

Jessie: What did it say?

Meowth: It said it's not interested in joining an evil team. It wants to stay here no matter what!

James: Why you want to stay here.

Gengar: Gen Gengar

After hearing Gengar's words, Meowth began to break in to tears.

Meowth: (crying) That's such a sad story!

James: Translation

Meowth: (crying) Gengar said that it made a promise to its old trainer that it would stay here and wait for him to return! It so loyal that it doesn't care if it has to wait a lifetime!

Jessie: Well if you're not going to join us willingly then we'll just capture you by force!

Meowth: Wait! You're going to catch Gengar by force!

Jessie: Of course! We came here here to catch a Shiny Gengar and we're not taking no for an answer! Go, Mimikyu!

Mimikyu: Kyu!

James: You too Cacnea!

Cacnea: Cacnea!

Jessie: Use Shadow Ball!

Mimikyu fired a Shadow Ball but Gengar dodge it and attack Mimikyu with Dark Pulse. 

James: Cacnea use Pin Missile!

Cacnea fired multiple Pin Missiles and Gengar managed to dodge most of them but a couple managed hit it. 

Jessie: Now use Shadow Claw! 

Mimikyu landed Shadow Claw on Gengar causing it to fall to the ground.

James: Now finish it with Needle Arm!

Cacnea was about to attack Gengar, who still recovering. Before Cacnea could land Needle Arm, suddenly everyone heard a voice.

Y/N: Riolu, Vacuum Wave!

Riolu's Vacuum Wave to hit Cacnea back and save Gengar. Y/N, Riolu, Goh, and Scorbunny arrive to protect Gengar.

Jessie: It's the twerps!

Y/N: Leave Gengar alone! It already his a trainer!

Goh: You have no right steal someone Pokemon!

James: Get lost! This Gengar is ours and we're taking it!

Y/N: We're not gonna let you steal it! Right Riolu! 

Riolu: olu!

Gengar got back up and looked at Y/N and Riolu. 

Gengar: Gengar?

Y/N: Don't worry Gengar. We'll protect you. 

Jessie: Use Shadow Ball!

James: Use Needle Arm!

Y/N: Riolu dodge and use Quick Attack on Cacnea!

Riolu: Rio!

Riolu dodged Mimikyu's Shadow Ball and hit Cacnea with Quick Attack.

James: Cacnea, Pin Missile go!

Riolu tried to dodge Pin Missile but took some damage.

Jessie: Mimikyu use Wood Hammer!

Riolu blocked Wood Hammer but still took heavy damage. 

Jessie: Now use Play Rough!

Mimikyu dashed toward Riolu to use Play Rough but was hit by a Shadow Ball, surprising everyone. Y/N turned around saw it Gengar who fired that Shadow Ball and saved Riolu.

Y/N: Gengar?

Gengar: Gengar Gen!

Y/N: You want to help us battle? 

Gengar: (nodding) Gengar!

Y/N: (smiling) Alright let do this together!

Riolu: (smiling) Riolu!

Jessie: Fraternizing with the twerps?  Mimikyu use Shadow Claw on Gengar!

James: Cacnea use Needle Arm on Riolu!

Y/N: Riolu use Counter

Riolu blocked Cacnea attack and used Counter to punch Cacnea to the ground. Mimikyu tried to attack with Shadow Claw but Gengar used  Phantom Force to vanish, dodging Shadow Claw. Gengar reappeared behind Mimikyu and attacked, dealing super effective damage.

Jessie: What was that?!

Y/N: Wow Gengar used Phantom Force!

Goh: That was awesome move.

Gengar then used Toxic on Cacnea, inflicting poison damage. 

James: Oh no! Cacnea!

Y/N: Let's wrap this up! Riolu, Vacuum Wave! Gengar, Shadow Ball!

Riolu and Gengar attacks Team Rocket with Vacuum Wave and Shadow Ball. The explosion caused the trio to blast off. 

Team Rocket: We're blasting off again!!!

Y/N: Great job Riolu.

Riolu: olu!

Y/N: You too Gengar. Thanks for your help.

Gengar: Gengar

Y/N took out the same folded paper he found in the trunk. 

Y/N: Gengar we have something to tell you. We found this letter in the trunk. It's a letter from your trainer.

Gengar: Gen?

Y/N read the letter to Gengar.

Dear Gengar,

If you're reading this, I am dying. I was diagnosed we a terminal disease and unfortunately it is incurable. I went to the Hospital to go into hospice care for the remaining of my life. I kept this a secret from you because I was too afraid to tell you. I just could bare to see you worried about me. I know I broke my promise to you but it my time has come to an end. It looks like you'll have to continue your life without me. I know parting ways can be painful but let me passing stop you from living your life. Goodbye my friend and remember even though I'm gone our friendship will always last a lifetime.

Love, Kirk.

After hearing Y/N read the letter, Gengar looked shocked for a couple second. The Ghost type sat down and started tearing up after finding out it will never be reunited with its trainer.

Gengar: (crying) Gengar!

Goh: Poor Gengar. That would be a shock to anyone. All it wanted was to be with its trainer again.

again.

Riolu and Scorbunny helped comfort the crying Gengar then Y/N bend down to one knee to talk to Gengar.

Y/N: I'm sorry for your loss Gengar. Truly I am. I understand why you are so upset. You were alone for the past three years. I can't imagine how sad you feel right now. But remember what your trainer said. You can't let his passing keep from living your life. 

Gengar stopped crying and looked at Y/N.

Y/N: Losing a friend a can be heartbreaking but he wouldn't want you to spent your whole life being sad. You have to keep moving forward. Look towards the future instead looking back on the past. 

Goh: That's right. You can't give up on life! There's so much out there for to see!

Y/N: And there are a lot of new people to make friends with! Like me!

Gengar: Gen?

Y/N extends his hand to Gengar.

Y/N: How about you join us? If you come with us you'll get to have all adventures, have lots of battles, and make all of new friends.

Riolu: (happily) Riolu!

Gengar looked at Y/N before closing its eyes and smiled as it made decision. 

Gengar: Gengar 

Gengar quickly phrased through the Cerise Lab, surprising Y/N and Goh. After waiting a few second, Gengar returned holding its Poke Ball that was in the old trunk. Gengar then handed Y/N its Poke Ball, signifying that it wants to join him.

Y/N: (smiling) Then it's settled! Welcome to the team Gengar!

Riolu: (smiling) Riolu!

Gengar: Gengar!

Gengar gave Y/N and Riolu a friendly hug as a thank you for letting it join them.

Y/N: Yeah! I caught a Shiny Gengar!

Y/N and Goh solved the mystery of the behind the haunted laboratory. They help Gengar out of its depression and it's now starting a new journey alongside Y/N. As the journey continues...

A/N: Gengar is a male. It has the Curse Body ability. It moveset includes Shadow Ball, Dark Pulse,  Psychic, Phantom Force, Toxic, and Hex.